Multimedia sound system with digital audio interface, comprising multimedia sound system equipped with USB external sound card or IEEE1394 external sound card is a prior art.
For instance, Microsoft launched a DSS 80 USB sound system powered by external power supply at the end of last century.
In 2006, PHILIPS released a DGX320 sound system with a 1-inch high pitch speaker, a 2-inch full frequency band speaker and a passive bass radiator built in each speaker. The sound system is powered by PC USB interface with an output power of 2×1 W and a frequency response range of 100 Hz to 20 KHz.
In March 2006, DN-MPS100 sound system introduced by the Japanese company, EVERGREEN, consists of two 3-inch caliber speakers, of which one is mounted on a USB interface on panel board, while one is mounted MP3 codec on the back side of the speaker and radio circuits inside the speaker. The audio frequency input power of the sound system is 2×7 W. The sound system adopts attached AC adaptor and is independently powered by the power network.
In January 2007, at the CES exhibition, YAMAHA released a NX-U10 USB sound system. Adopting two 1.5-inch mini speakers, the sound system store the energy provided by PC USB interface of a notebook in a set of capacitor plates through “charging capacitor amplifier” technology and SR-Bass bass sound enhancement technology so as to satisfy the explosive demand of power handling when low audio frequency is output intensely. The frequency response range of the sound system is 90 Hz to 20 KHz.
However, a large number of mini USB sound systems based on a USB digital audio codec (DAC), 2 to 4 mini speakers, and a set of digital or analog output amplifiers, had ever flooded both domestic and foreign markets. On the whole, these products can not satisfy consumers' demands due to their poor sound quality and small sound volume.
It is known that the output voltage of PC USB interface is DC 5 volts (±5%) and the output current is 500 mA, which means each USB interface can only provide rated continuous power of 2.5 W. However, the rated continuous input power of a 2-inch caliber speaker is 2 W to 4 W and the SPL value can be as high as 83 dB/1 W/1 m. The rated continuous input power of a 1.25-inch to 1.5-inch speaker is roughly 1 W to 3 W and the SPL value is normally 79 to 82 dB/1 W/1 m. The rated continuous input power of a 3-inch caliber speaker is approximately 5 W to 8 W and the SPL value can hardly exceed 85 dB/1 W/1 m. Although the 3-inch speaker has a lower resonance frequency Fo, its electronic acoustic recovery quality of low audio frequency band is much better than mini speaker with a diameter that is less than 2 inches. Nevertheless, audio manufacturers would rather choose 2-inch caliber or smaller speakers with relatively poor sound quality as the sound unit of USB sound. That is because approximate 2×1 W electric power output by power amplifier of computer's USB sound system can hardly drive a pair of 3-inch caliber speakers as well as ensure ample sound volume and good sound quality.
Generally speaking, the electro-acoustic conversion efficiency of speakers with a 2-inch diameter is only 0.125%. That is to say: when the speaker's input electrical power is 1 W, only 0.00125 W electric energy is converted into useful sound energy; the remaining 0.99 W electric energy is converted into harmful thermal energy which people do not want and is completely wasted. Compared with the 7% energy conversion efficiency of incandescent lamp, the efficiency of 2-inch speaker is only the lamp's 1/56.
Therefore, moving-coil speaker (hereinafter referred to as speaker), the mainstream speaker, is the most widely used as well as the least efficient electrical products in human society worldwide in the 20th and 21st century.
Firstly, except for U.S. Pat. No. 5,748,760, almost all commercialized speakers worldwide have only one magnetic gap and one coil. When the coil is communicated with sound frequency current, according to Fleming's left hand rule, under the interaction of magnetic gap field, the coil will generate an electrical power F which drives the speaker's diaphragm to carry out reciprocating piston movement; the speaker will produce sound due to vibration of air. However, when the coil is going on reciprocating piston movement, the permanent magnetic force line in the magnetic gap will cut across the coil vertically, generating a dynamo electric potential by induction based on Fleming's right hand rule, that is what we call counter electromotive force. And the counter electromotive force will superpose the audio frequency signal in the speaker's coil, consequently leading to speaker distortion. In general, the larger the vibration amplitude of speaker's coil is, the larger the acceleration is, and the lower the frequency of audio signaling current is, the larger the amplitude of the counter electromotive force will be and the greater the distortion will be. Thus, constrained by distortion of counter electromotive force, the prior art is unlikely to enhance a speaker's efficiency. Secondly, although the prior art can be successfully applied to a full range speaker of medium or small caliber, the prior art cannot a full range speaker with a performance price that would be accepted by vast consumers and be put into mass industrial production. The fundamental reason lies in the fact that a speaker's impedance is a function of the speaker's working current frequency: the lower the frequency is, the lower the impedance will be; the higher the frequency is, the higher the impedance will be. For every traditional speaker having a single magnetic gap and a single coil, the effective value of high audio frequency current in the coil is much smaller than that of low audio frequency current. Thus, the sound pressure generated at the high audio frequency band of the speaker is less than that at the low audio frequency band. For a speaker with a caliber of less than 3 inches, we can make up for the defect through technical means since the mass of speaker's vibration system is relatively small But the resonant frequency Fo of this sort of small-caliber speaker is necessarily higher, thus it is difficult to achieve satisfactory electronic acoustic recovery effect of low audio frequency. For speakers with a caliber of more than 3 inches, the increase in unit diameter will cause Fo to drop towards low audio frequency, thus improving the speaker's low audio frequency recovery quality. But, the mass of the speaker's vibration system will increase; the speaker's SPL value of high audio frequency band will drop dramatically at the 5 to 10 KHz or above frequency band. Therefore, a high pitch speaker and a medium and bass speaker are usually mounted in the prior art multimedia sound system to satisfy customers' full range demand of Fo to 20 KHz. In order to coordinate work of the two speakers, a frequency divider must be added. But the use of frequency divider will further lead to the sound system's energy consumption and distortion. Thirdly, U.S. Pat. No. 5,748,760 can not be applied to medium and small caliber speakers, such as the most widely used 0.5-inch to 8-inch caliber inside a magnet field speaker.
Fourthly, in spite of these, the USB sound of PC still has some widely acknowledged advantages. For instance, 1) the adoption of external sound card can completely get rid of the electromagnetic wave interference generated by transformer and electric fan inside computer case; 2) in terms of anti-interference, amplifier efficiency, signal to noise ratio and dynamic range, codec and coder based on PCM/PWM audio digital technology, D type audio power amplifier or T type audio power amplifier based on the 1 Bit Δ-Σ digital analog converter technology of DPPTM patent perform much better than complex wave filter, R/C blocking network and AB type power amplifier of simulated audio system; 3) Until now, the digital-analog or analog-digital conversional accuracy of mainstream sound card of computer's PCI is only 16 Bit, but the USB sound card's DAC of many mini USB audio often have an conversional accuracy of 20 Bit; 4) although the class of USB's universal serial bus is lower than PCI's parallel bus, the digital multimedia audio system characterized by USB/IEEE1394 (including derivative products such as 1394.b) interface technique will become the inevitable development trend of the future due to continuous improvement of CPU's performance, especially the emergency of CPU with dual or four core processors and the continuous enhancement of CPU function, the popularization of 2.0 USB interface standard as well as the forthcoming 3.0 USB interface standard.
For this reason, from 1997 the inventor started to put forward a series of invention patents with an attempt to change the above extremely unreasonable situation. They mainly consist of: A) authorized patents of the WO 01/15493 patent family including CN00122197.3, U.S. Pat. No. 6,795,564, CN200520035371.X, CN200620033128.9 and disclosed patent applications including JP publication 2003-531508, US20050099255, CN200510091936.0 and CN200610020317.7; B) authorized patents of the WO 99/31931 patent family including CN99114781.2,TW88109796 and CN00222469.0; C) undisclosed patent applications including CN200710181973.X, CN200710123821.4, CN200720126871.3, CN200810065384.X and CN200820092177.9.
For detailed information, please refer to the above disclosed patent literature.